Characterization of a DC glow discharge in N2-H2 with electrical measurements and neutral and ion mass spectrometry

The addition of small amounts of H2 were investigated in a DC glow discharge in N2, at low pressure (~1 mbar) and low power (0.05 to 0.2 W.cm-3). We quantified the electric field, the electron density, the ammonia production and the formation of positive ions for amounts of H2 varying between 0 and 5%, pressure values between 0.5 and 4 mbar, and currents between 10 and 40 mA. The addition of less than 1% H2 has a strong effect on the N2 plasma discharges. Hydrogen quenches the (higher) vibrational levels of N2 and some of its highly energetic metastable states. This leads to the increase of the discharge electric field and consequently of the average electron energy. As a result, higher quantities of radical and excited species are suspected to be produced. The addition of hydrogen also leads to the formation of new species. In particular, ammonia and hydrogen-bearing ions have been observed: N2H+ and NH4+ being the major ones, and also H3+, NH+, NH2+, NH3+, N3H+ and N3H3+. The comparison to a radiofrequency capacitively coupled plasma (RF CCP) discharge in similar experimental conditions shows that both discharges led to similar observations. The study of N2-H2 discharges in the laboratory in the adequate ionization conditions then gives some insights on which plasma species made of nitrogen and hydrogen could be present in the ionosphere of Titan. Here, we identified some protonated ions, which are reactive species that could participate to the erosion of organic aerosols on Titan.Comment: Paper accepted in Plasma Sources Science and Technology in March 2023. The current version on arXiv is the submitted versio

Étude des mécanismes fondamentaux des plasmas de CO2

The use of non thermal plasmas is one of the most promising paths to efficiently recycle CO2 into more complex organic molecules, such as energy-dense hydrocarbon fuels, and it is compatible with the use of intermittent renewable energy sources. To obtain satisfactory energy yields, it is necessary to properly control the energy transfer processes, including the vibrational energy of the CO2 believed to be beneficial for the CO2 conversion, or the energy stored in electronically excited species. Recombination processes producing CO2 from the dissociation products (the so-called back reaction) must also be prevented. However, despite the extensive literature in the fields of CO2 lasers, atmospheric entry plasmas or CO2 conversion, many of the basic mechanisms essential for the description of CO2 plasmas are still very poorly understood. The objective of this thesis is therefore to perform experiments under sufficiently well controlled conditions to identify and study some of these fundamental mechanisms. Two types of plasma sources, a DC "glow" discharge and a radio frequency (RF) discharge were studied at low pressures (27-1000 Pa) to slow down characteristic times of various processes. Advanced optical diagnostic techniques were used in situ and time-resolved to obtain all the relevant parameters for a complete description of the plasma. The densities and vibrational temperatures of CO2 and CO were measured by infrared absorption spectroscopy (FTIR), giving also insight in back reaction mechanisms. The density and loss frequencies of oxygen atoms were obtained with High Resolution Two photon Absorption Laser Induced Fluorescence (HR-TALIF), actinometry and Cavity Ring Down Spectroscopy (CRDS), while isotopic exchange measurements provided information on the role of O(1D). Most of these techniques were also used to determine the gas temperature showing simultaneously the consistency and accuracy of the different techniques.The experimental results made possible, for instance, the identification of the most accurate cross section for CO2 dissociation by electronic impact or the quantification of the vibratory de-excitation of CO2 by oxygen atoms. The obtained data were also used to validate a 0D kinetic model developed at IST Lisbon, which allowed the validation of the rates for vibration-vibration or vibration-translation energy transfer processes for the low vibrational levels of CO2.Another important part of the work focused on the role of the surfaces on the CO2 plasma kinetics. The O atoms loss processes were found to be dominated by surface recombination, dependent on the temperature of the O atoms near the surface, similarly to a pure O2 plasma. However, it was found that CO2 plasma can passivate SiO2 surfaces, reducing the recombination probability of oxygen atoms at the walls, and making it identical under plasma exposure and in post-discharge, unlike what is observed in O2 plasma. A preliminary comparison with a Monte-Carlo surface model, provides a valuable insight in the surface mechanisms involved. Large specific SiO2 surfaces were found to induce CO2 formation in the surface under high O atom flux regimes, limiting dissociation efficiency, whereas the use of carbon-based surfaces showed an enormous potential to use the oxygen atoms to enhance the final CO2 conversion, demonstrating the key role of the surfaces in the efficiency of the CO2 conversion and the importance of a proper handling of the oxygen atoms. These results are therefore very valuable to understand which materials would be relevant to be used as catalysts to improve CO2 conversion efficiency by plasma.The thesis provides a detailed view on the fundamental mechanisms controlling the kinetics of CO2 plasmas, and the results presented are therefore useful not only for developing more efficient CO2 conversion processes, with or without catalysts, but they are also relevant in fields such as surface treatment using O2/CO2-containing plasmas.Les plasmas froids constituent l’une des voies les plus prometteuses pour recycler efficacement le CO2 en carburants de synthèse ou en molécules de base pour la chimie organique « verte ». Pour obtenir des rendements énergétiques satisfaisants, il est nécessaire de bien contrôler les transferts d’énergie vibrationnelles dans la molécule de CO2 considérés bénéfiques pour la dissociation, mais aussi l’énergie transférée aux états électroniquement excités ainsi que les processus de recombinaison de O avec CO. Toutefois, en dépit d’une littérature conséquente sur les lasers CO2 et les plasmas de rentrée atmosphérique, de nombreux processus essentiels à la description des plasmas de CO2 sont encore très mal compris. Cette thèse a donc pour objectif de réaliser des mesures dans des conditions suffisamment bien contrôlées pour identifier et étudier certains de ces mécanismes. Pour y parvenir Deux types de sources de plasma, une décharge "luminescente" DC et une décharge radiofréquence (RF) ont été étudiées à basse pression (27-1000 Pa) ce qui ralentit les temps caractéristiques des différents processus. Ces sources plasmas ont été étudiées à l’aide de techniques optiques de pointe complémentaires afin d’obtenir dans un même système, tous les paramètres pertinents pour une description complète du plasma. Ainsi les densités et températures vibrationelles de CO2 et CO ont été mesurées par spectroscopie d’absorption infrarouge (FTIR), la densité et les fréquences de perte des atomes d’oxygène par TALIF haute résolution, actinométrie et CRDS, alors que des mesures d’échanges isotopiques ont donné des informations notamment sur le rôle de O(1D). La plupart de ces techniques permettent également de déterminer la température du gaz illustrant simultanément la précision et la cohérence des mesures obtenues par différentes techniques.Toutes ces mesures ont pu être réalisées in situ et résolues en temps pour suivre l’évolution de ces paramètres durant et après des pulses du plasma, que ce soit en DC glow ou en RF. Outre la validation de modèles cinétiques développés à l’IST Lisbonne, toutes ces données ont permis d’obtenir de nombreux résultats parmi lesquels l’identification de la meilleure section efficace de dissociation du CO2 par impact électronique, ou la quantification de la désexcitation vibrationelle du CO2 par les atomes d’oxygène.Un aspect important du travail a également porté sur l’influence des surfaces sur le plasma. Les processus de perte d'atomes de O dominés par la recombinaison en surface, se sont avérés dépendre de la température des atomes près de la surface. Il a aussi été montré qu’un plasma de CO2 peut passiver la surface de SiO2 diminuant la probabilité de recombinaison des atomes d’oxygène aux parois, et la rendant identique sous exposition au plasma et en post-décharge contrairement a ce qui est observé en plasma de O2. Une comparaison préliminaire avec un modèle de surface Monte-Carlo donne un bon éclairage sur les mécanismes de surface impliqués. En utilisant des fibres de SiO2 de grandes surfaces spécifiques, la formation de CO2 en surface a été mise en évidence ce qui limite l'efficacité de dissociation. Au contraire des surfaces à base de carbone ont permis une augmentation importante de la conversion du CO2 en piégeant les atomes d'oxygène. Ceci démontre le rôle essentiel que des surfaces catalytiques pourraient jouer dans l’efficacité de conversion du CO2 et l'importance de contrôler la réactivité des atomes d'oxygène.L’ensemble de ces résultats offre une vision beaucoup plus détaillée de la cinétique des plasmas de CO2. Il s’agit donc d’un travail utile non seulement pour développer de nouveaux procédés efficaces de conversion du CO2 avec ou sans catalyseurs, mais également dans des domaines tels que les traitements de surfaces utilisant des plasmas contenant du CO2 ou les problèmes d’entrée atmosphériques par exemple

Time Evolution of the Dissociation Fraction in rf CO 2 Plasmas: Impact and Nature of Back-Reaction Mechanisms

International audienc

Fast O Atom Exchange Diagnosed by Isotopic Tracing as a Probe of Excited States in Nonequilibrium CO 2 –CO–O 2 Plasmas

International audienc

Excitation and relaxation of the asymmetric stretch mode of CO<SUB>2</SUB> in a pulsed glow discharge

International audienceThe excitation and relaxation of the vibrations of CO2 as well as the reduction of CO2 to CO are studied in a pulsed glow discharge. Two diagnostics are employed, being (1) time-resolved in situ Fourier transform infrared (FTIR) spectroscopy and (2) spatiotemporally resolved in situ rotational Raman spectroscopy. Experiments are conducted within a pressure range of 1.3-6.7 mbar and a current range of 10-50 mA. In the afterglow, the rate of exponential decay from the asymmetric stretch temperature (T3) to the rotational temperature (Trot) is found to be only dependent on Trot, in the conditions under study. The decay rate &#961;T3-Trot follows the relation &#961;T3-Trot = 388 s-1 exp((Trot - 273 K)/(154 K)). Pressure and varying concentrations of CO and (presumably) atomic oxygen did not show to be of significant influence. In the active part of the discharge the excitation of T3 showed to be positively related to current and negatively to pressure. However, the contribution of current to vibrational excitation is ambiguous: the conversion of CO2 and therefore the fraction of CO in the discharge, is found to be strongly dependent on the current, with a conversion factor of 0.05 to 0.18 for 10 mA to 50 mA, while CO can contribute to the excitation through near-resonant collisions. A clear relation between the elevation of T3 and the dissociation of CO2 could not be confirmed, though conversion peaks are observed in the near afterglow, which motivate future experiments on vibrational ladder-climbing directly after termination of the discharge

13th Frontiers in Low temperature plasma diagnostics (FLTPD)

International audienc

8th International Workshop on Plasma Spectroscopy (IPS)

International audienc

Characterization of a DC glow discharge in N₂-H₂ with electrical measurements and neutral and ion mass spectrometry

International audienceThe addition of small amounts of H2 were investigated in a DC glow discharge in N2, at low pressure (~1 mbar) and low power (0.05 to 0.2 W.cm-3). We quantified the electric field, the electron density, the ammonia production and the formation of positive ions for amounts of H2 varying between 0 and 5%, pressure values between 0.5 and 4 mbar, and currents between 10 and 40 mA.&#xD;The addition of less than 1% H2 has a strong effect on the N2 plasma discharges. Hydrogen quenches the (higher) vibrational levels of N2 and some of its highly energetic metastable states. This leads to the increase of the discharge electric field and consequently of the average electron energy. As a result, higher quantities of radical and excited species are suspected to be produced. The addition of hydrogen also leads to the formation of new species. In particular, ammonia and hydrogen-bearing ions have been observed: N2H+ and NH4+ being the major ones, and also H3+, NH+, NH2+, NH3+, N3H+ and N3H3+.&#xD;The comparison to a radiofrequency capacitively coupled plasma (RF CCP) discharge in similar experimental conditions shows that both discharges led to similar observations. The study of N2-H2 discharges in the laboratory in the adequate ionization conditions then gives some insights on which plasma species made of nitrogen and hydrogen could be present in the ionosphere of Titan. Here, we identified some protonated ions, which are reactive species that could participate to the erosion of organic aerosols on Titan

13th Frontiers in Low temperature plasma diagnostics (FLTPD)

International audienc

Characterization of a DC glow discharge in N₂-H₂ with electrical measurements and neutral and ion mass spectrometry

International audienceThe addition of small amounts of H2 were investigated in a DC glow discharge in N2, at low pressure (~1 mbar) and low power (0.05 to 0.2 W.cm-3). We quantified the electric field, the electron density, the ammonia production and the formation of positive ions for amounts of H2 varying between 0 and 5%, pressure values between 0.5 and 4 mbar, and currents between 10 and 40 mA.&#xD;The addition of less than 1% H2 has a strong effect on the N2 plasma discharges. Hydrogen quenches the (higher) vibrational levels of N2 and some of its highly energetic metastable states. This leads to the increase of the discharge electric field and consequently of the average electron energy. As a result, higher quantities of radical and excited species are suspected to be produced. The addition of hydrogen also leads to the formation of new species. In particular, ammonia and hydrogen-bearing ions have been observed: N2H+ and NH4+ being the major ones, and also H3+, NH+, NH2+, NH3+, N3H+ and N3H3+.&#xD;The comparison to a radiofrequency capacitively coupled plasma (RF CCP) discharge in similar experimental conditions shows that both discharges led to similar observations. The study of N2-H2 discharges in the laboratory in the adequate ionization conditions then gives some insights on which plasma species made of nitrogen and hydrogen could be present in the ionosphere of Titan. Here, we identified some protonated ions, which are reactive species that could participate to the erosion of organic aerosols on Titan